1. Field of the Invention
The present invention relates to an optical semiconductor device having both light emission and light reception abilities for use in optical information processing, optical measurement, optical communication, and the like.
2. Prior Art
As optical semiconductor device for use in optical information processing, optical measurement, optical communication, and the like, the one in which a light source and a light-receptive part (photodetector) are mounted in the same package has found use in recent years.
Below, a conventional optical semiconductor device will now be described. FIG. 13 is a plan view schematically showing the plane layout of a conventional optical semiconductor device. FIG. 14 is a schematic cross-sectional view taken on line A-A' of a semiconductor substrate 1 shown in FIG. 13. Further, FIG. 15 is a schematic cross-sectional view taken on line B-B' of the semiconductor substrate 1 shown in FIG. 13.
Referring now to FIGS. 13, 14, and 15, the semiconductor substrate 1 is composed of, for example, Si, and it is provided with a rectangular concave portion 1a on the surface. A semiconductor laser element 2 is composed of, for example, GaAs, and it serves as light source for emitting signal detection light. The semiconductor laser element 2 is mounted at the concave portion 1a of the semiconductor substrate 1 so that the optical axis of the signal detection light is generally in parallel relationship with the surface of the semiconductor substrate 1, thus to be integral with the semiconductor substrate 1. Specifically, the semiconductor laser element 2 is fixed on the underside of the concave portion 1a.
The above-described concave portion 1a is so configured as to reflect the signal detection light of the semiconductor laser element 2 by one inclined side thereof in a direction substantially perpendicular to the surface of the semiconductor substrate 1. That is, one inclined side of the concave portion 1a becomes a reflection surface. Further, the one electrode for applying a voltage to the semiconductor laser element 2, which is not shown, is formed at the region, on which the semiconductor laser element 2 is mounted, in the underside of the concave portion 1a. Whereas the other electrode is formed on the opposite one of the surface of the semiconductor laser element 2 in contact with the underside of the concave portion 1a.
Each one of the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 is comprised of an impurity diffusion area composed of, for example, Si, and it serves as light-receptive part. It is formed in the semiconductor substrate 1 by impurity diffusion. The light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 are selectively formed in the peripheral area of the concave portion 1a on the surface of the semiconductor substrate 1, for example, in the area lateral to the concave portion 1a taking the direction of emission of the signal detection light from the semiconductor laser element 2 as forward direction on the surface of the semiconductor substrate 1. Thus, they receive return light from an optical recording medium.
The above-described semiconductor substrate 1 and each one of the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 are mutually opposite in conductivity type. Between the semiconductor substrate 1 and the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 is applied such a voltage as to result in a reverse bias.
A monitor area 12 comprises an impurity diffusion area composed of, for example, Si, and it is provided in the backward direction of the concave portion 1a on the surface of the semiconductor substrate 1. The quantity of signal detection light from the semiconductor laser element 2 is detected by the monitor area 12.
The above-described semiconductor substrate 1 and the monitor area 12 are mutually opposite in conductivity type. Between the semiconductor substrate 1 and the monitor area 12 is applied such a voltage as to result in a reverse bias. The impurity concentration of the monitor area 12 is set so as to be comparable to that of the portions of the respective light receiving elements for signal detection 3, 4, 5, 6, 7, and 8.
With this optical semiconductor device, the signal detection light is emitted from the semiconductor laser element 2 substantially in parallel with the surface of the semiconductor substrate 1 as shown by an arrow 9 in FIG. 15. Then, the signal detection light is reflected by the inclined surface of the concave portion 1a existing in front thereof in a direction substantially perpendicular to the surface of the semiconductor substrate 1, thus to be applied onto an object of signal detection such as optical recording medium.
In this process, the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 are formed at the positions deviating from the direction of emission of signal detection light from the semiconductor laser element 2. For example, the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 are formed in the area lateral to the concave portion 1a taking the direction of emission of the signal detection light from the semiconductor laser element 2 as forward direction, in the surface of the semiconductor substrate 1. This is achieved for preventing the situation as follows: that is, the signal detection light emitted from the semiconductor laser element 2 enters the semiconductor substrate 1, resulting in the occurrence of carriers, which adversely affect each signal detection level of the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8. It is noted that, each position of the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 may properly be in the peripheral area of the semiconductor laser element 2, and that the number thereof may properly be one or more.
For the optical semiconductor device configured as described above, the operation thereof will now be described below. First, the signal detection light emitted from the semiconductor laser element 2 is converged on an optical recording medium (not shown) through an objective lens (not shown). Then, the light corresponding to the signal of the optical recording medium is reflected therefrom to become return light, which is then converged on the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8. Consequently, optical signals are outputted from the respective light receiving elements for signal detection 3, 4, 5, 6, 7, and 8. In this process, the quantity of the signal detection light emitted from the semiconductor laser element 2 is being monitored in the monitor area 12, during which the semiconductor laser element 2 is controlled so that the value is made constant.
Incidentally, with the above-described prior art configuration, signal detection light is emitted from the semiconductor laser element 2 in a direction shown by the arrow 9. In addition, unnecessary light other than the signal detection light (below, referred to as stray light) occurs.
Below, a concrete description will now be given to stray light. The light in association with the optical semiconductor device includes laser light emitted from the semiconductor laser element 2, and return light resulting from reflection from a medium such as optical disk, or magneto-optic disk. However, it is the stray light resulting from the laser light emitted from the semiconductor laser element 2 that matters in the present invention
The laser light emitted from the semiconductor laser element 2 includes the light emitted from the front and the light emitted from the rear, and also includes the effective light and the ineffective light for signal detection.
The laser light, including both the light from the front, and the light from the rear of the semiconductor laser element 2, is generally emitted extending 180 degrees both vertically and horizontally. In this step, the larger the output angle of the laser light is, the smaller the light quantity thereof is.
In the laser light having the above-described emitting characteristics, the light emitted forward from the front of the semiconductor laser element 2 is effective for signal detection, which serves as signal detection light. On the other hand, the light emitted in directions other than the forward direction, specifically, in an oblique direction or just sideward is ineffective, which results in stray light. when the semiconductor substrate 1 is irradiated with the stray light, stray light carriers occur in the semiconductor substrate 1.
On the other hand, the light emitted backward out of the light emitted from the rear of the semiconductor laser element 2 is applied in the vicinity of the monitor area 12. The light emitted backward is used for detecting the quantity of light from the semiconductor laser element 2. Further, the light emitted in directions other than the backward direction, specifically, in an oblique direction or just sideward results in stray light. The irradiation of the underside, or sides of the concave portion 1a of the semiconductor substrate 1 with the stray light results in the occurrence of carriers in the semiconductor substrate 1. The carriers are partially captured by the monitor area 12, while most of the remainder results in stray light carriers, which adversely affect the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8.
The front of the semiconductor laser element 2 lies in extremely close proximity to the inclined side (front side) of the concave portion 1a. Accordingly, most of the stray light emitted from the front impinges on the inclined side of the concave portion 1a. Consequently, the stray light carriers resulting from the stray light emitted from the front less affect the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8.
However, the rear of the semiconductor laser element 2 and the rear side of the concave portion 1a are spaced at a relatively large distance from each other. Accordingly, the stray light emitted from the rear of the semiconductor laser element 2 tends to be applied onto the underside, or the sides of the concave portion 1a. The underside and the sides of the concave portion 1a are at a short distance from the positions where the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8 are formed. Consequently, the straylight carriers occurred in the vicinity of the underside and the sides of the concave portion 1a tend to adversely affect the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8.
In other words, the stray light emitted from the rear of the semiconductor laser element 2 is directly or indirectly applied onto the surface, especially the underside or the sides of the concave portion 1a, of the semiconductor substrate 1. This results in the occurrence of stray light carriers around the concave portion 1a on the surface of the semiconductor substrate 1.
The stray light carriers resulting from the stray light applied onto the sides of the concave portion 1a on the surface of the semiconductor substrate 1 are found to be around the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8. Similarly, the stray light carriers resulting from the stray light applied onto the underside of the concave portion 1a are also found to be around the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8. This results in the following problems: that is, the stray light carriers are absorbed by the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8, which causes the optical signals of larger level than actual signal level to be outputted from the light receiving elements for signal detection 3, 4, 5, 6, 7, and 8.